1. Field of the Invention
The present invention relates to a webbing winding device which structures a seat belt apparatus for restraining, with a long belt-form webbing belt, a body of an occupant sitting on a seat of a vehicle or the like.
2. Description of the Related Art
A seat belt apparatus, which restrains a body of an occupant sitting at a seat of a vehicle with a webbing belt, is equipped with a webbing winding device fixed to a body of the vehicle sideward of the seat. The webbing winding device is equipped with, for example, a spool (winding shaft) whose axial direction is substantially along a front-rear direction of the vehicle. A length direction base end side of the webbing belt is anchored at the spool. The spool can wind the webbing belt on in a layered form at an outer peripheral portion of the spool. Thus, when the seat belt apparatus is not in use, the webbing belt can be wound on and accommodated at the outer peripheral portion of the spool.
An urging member such as a spiral spring or the like is provided at the webbing winding device, and urges the spool in a winding direction for winding up the webbing belt. The webbing belt is wound on and accommodated by urging force of this urging member, and slackness and the like of the webbing belt is eliminated by the urging force of the urging member in a state in which the webbing belt is applied to the body of the occupant.
Further, a mechanism has been considered in which, at a time of rapid deceleration of the vehicle or the like, a slight looseness known as “slack” or the like is eliminated and a restraining force from the webbing belt on the body of the occupant is increased, by winding the webbing belt onto the winding shaft by a certain amount. Thus, the body of the occupant is more assuredly held. This type of mechanism often has a structure which detects the state of rapid deceleration of the vehicle with an acceleration sensor, and forcibly rotates the winding shaft in the winding direction on the basis of an electronic signal from the acceleration sensor.
Meanwhile, a structure has also been considered which detects a distance forward to another vehicle, an obstacle or the like with a distance sensor or the like and, when the distance forward to the vehicle, obstacle or the like falls below a certain value, drives a motor and rotates the winding shaft in the winding direction by rotating force of the motor.
Accordingly, in the case of a structure which rotates the winding shaft in the winding direction by rotating force of the motor, a “one-way clutch” is interposed between an output shaft of the motor and the winding shaft. This one-way clutch is generally structured to couple the output shaft with the winding shaft only when the output shaft of the motor rotates in a direction for winding the webbing belt onto the winding shaft. Thus, at a time of usual winding of the webbing belt onto the winding shaft, a time of drawing out the webbing belt from the winding shaft and the like, rotation of the winding shaft is not transmitted to the output shaft of the motor.
Incidentally, improving restraining force from the webbing belt and more assuredly holding the body of the occupant is preferable when a distance forward to a vehicle, obstacle or the like is less than a certain value. More specifically however, in a case in which a distance forward to a vehicle, obstacle or the like is sufficiently large, if the webbing belt restrains the body of the occupant with a large force, the occupant feels an excessive sensation of pressure, which is not preferable.
Accordingly, a structure has been considered which, when a distance forward to a vehicle, obstacle or the like becomes sufficiently large after having been in the above-described state in which the distance forward to the vehicle, obstacle or the like is less than the certain value, rotates the winding shaft by a certain amount in the drawing out direction by driving force of a motor, slightly loosens the webbing belt in its applied state, and alleviates the sensation of pressure.
Now, both in this structure which slightly loosens the webbing belt in the applied state and alleviates the sensation of pressure and in the structure which rotates the winding shaft by a certain amount in the winding direction to raise the restraining force from the webbing belt in a state of rapid deceleration, driving force of a motor is used. Thus, it is easy to devise a structure that is capable of realizing these two functions by forward-rotation and reverse-rotation of a motor.
However, as described above, the one-way clutch, which couples the output shaft of the motor and the winding shaft in the state of rapid deceleration of the vehicle, is a structure which does not couple the output shaft of the motor with the winding shaft when the winding shaft rotates or the output shaft rotates in the direction opposite to the direction for winding the webbing belt on the winding shaft. Therefore, it is necessary to add an extra clutch mechanism, to couple the output shaft of the motor with the winding shaft when the output shaft rotates in the direction opposite to the direction for winding up the webbing belt. Hence, the clutch mechanism becomes large and complicated.